Maybe all he wants is to make his lock bits work.

The solution is probably in the instruction manual for his programmer. (If he can find it)

BTW, a simple way to encrypt data is to xor the data with a "key" string in a recirculating shift register. That's easily done with software or a small PLD. It's important that both encryption and decryption processes be in precise synchronization, however.

A more sophisticated way is to do that, and feed back, an occasional bit of the cypher, selected by, say the last three bits of the cypher. That can easily be done with what's essentially a recirculating shift register, an XOR, and a 74151 data selector feeding yet another XOR. So long as both ends are in sync and use the same key, it's easy to decrypt. It's not so easy if you lose the key or the synchronizatoin.

Neither of these simple schemes provides a 1:1 mapping from clear code to cypher and back again.

How you do all that in an MCU depends entirely on the resources it provides. Maxim/Dallas has long pushed a line of "secure" processors that claim to support encryption.

RE